fossilfandomcom-20200214-history
La Brea Tar Pits
The La Brea Tar Pits (or Rancho La Brea Tar Pits) are a group of tar pits around which Hancock Park was formed, in urban Los Angeles. Natural asphalt (also called asphaltum, bitumen, pitch or tar - brea in Spanish) has seeped up from the ground in this area for tens of thousands of years. The tar is often covered with dust, leaves, or water. Over many centuries, the bones of animals that were trapped in the tar were preserved. The George C. Page Museum is dedicated to researching the tar pits and displaying specimens from the animals that died there. The La Brea Tar Pits are a registered National Natural Landmark. Location and Formation The La Brea Tar Pits and Hancock Park are situated within what was once the Mexican land grant of Rancho La Brea, now part of urban Los Angeles in the Miracle Mile district, adjacent to the Los Angeles County Museum of Art and the Craft and Folk Art Museum. Tar pits are composed of heavy oil fractions called asphaltum, which seeped from the earth as oil. In Hancock Park, crude oil seeps up along the 6th Street Fault from the Salt Lake Oil Field, which underlies much of the Fairfax District north of the park.2 The oil reaches the surface and forms pools at several locations in the park, becoming asphalt as the lighter fractions of the petroleum biodegrade or evaporate. This seepage has been happening for tens of thousands of years. From time to time, the asphalt would form a deposit thick enough to trap animals, and the surface would be covered with layers of water, dust, or leaves. Animals would wander in, become trapped, and eventually die. Predators would enter to eat the trapped animals and also become stuck. The tar pits visible today are actually from human excavation. The lake pit was originally an asphalt mine. The other pits visible today were produced between 1913 and 1915, when over 100 pits were excavated in search of large mammal bones. Various combinations of asphaltum and water have since filled in these holes. Normally, the asphalt appears in vents, hardening as it oozes out, to form stubby mounds. These can be seen in several areas of the park. As the bones of dead animals sink into the asphalt, it soaks into them, turning them a dark-brown or black color. Lighter fractions of petroleum evaporate from the asphalt, leaving a more solid substance, which encases the bones. Dramatic fossils of large mammals have been extricated from the tar, but the asphalt also preserves microfossils: wood and plant remnants, rodent bones, insects, mollusks, dust, seeds, leaves, and even pollen grains. Examples of some of these are on display in the museum.which? Radiometric dating of preserved wood and bones has given an age of 38,000 years for the oldest known material from the La Brea seeps. The pits still ensnare organisms today, so most of the pits are fenced to protect humans and animals. History The Native American Chumash people living in the area distinguished themselves as the finest boat builders among the California Natives. Pulling fallen Northern California redwood trunks and pieces of driftwood from the Santa Barbara Bay, Chumash ancestors soon learned to seal the cracks between the boards of the large wooden plank canoes using the natural resource of tar. This unique and innovative form of transportation allowed access up and down the coastline and on the Channel Islands. The Portolà expedition, a group of Spanish explorers led by Gaspar de Portolá, made the first written record of the tar pits in 1769. Father Juan Crespí wrote, "While crossing the basin the scouts reported having seen some geysers of tar issuing from the ground like springs; it boils up molten, and the water runs to one side and the tar to the other. The scouts reported that they had come across many of these springs and had seen large swamps of them, enough, they said, to caulk many vessels. We were not so lucky ourselves as to see these tar geysers, much though we wished it; as it was some distance out of the way we were to take, the Governor Portolá did not want us to go past them. We christened them Los Volcanes de Brea "the Tar Volcanoes".3 Harrison Rogers, who accompanied Jedediah Smith on his 1826 expedition to California, was shown a piece of the solidified asphalt while at Mission San Gabriel, and noted in his daybook (journal) that "The Citizens of the Country make great use of it to pitch the roofs of their houses".4 For some years, tar-covered bones were found on the Rancho La Brea property but were not initially recognized as fossils, because the ranch had lost various animals, including horses, cattle, dogs, and even camels, whose bones closely resemble several of the fossil species. The original Rancho La Brea land grant stipulated that the tar pits be open to the public for the use of the local Pueblo. Initially, they mistook the bones in the pits for the remains of pronghorn antelope (Antilocapra americana) or cattle that had become mired. Union Oil geologist W. W. Orcutt is credited with first recognizing that fossilized prehistoric animal bones were preserved in pools of asphalt on the Hancock Ranch in 1901. In commemoration of Orcutt's initial discovery, paleontologists named the La Brea coyote (Canis orcutti) in his honor Scientific Resources Category:Paleontological sites of North America